1. Field of the Invention
The invention relates to a process and apparatus for the regeneration of fluidized catalytic cracking catalyst.
2. Description of Related Art
In the fluidized catalytic cracking (FCC) process, catalyst, having a particle size and color resembling table salt and pepper, circulates between a cracking reactor and a catalyst regenerator. In the reactor, hydrocarbon feed contacts a source of hot, regenerated catalyst. The hot catalyst vaporizes and cracks the feed at 425C-600C, usually 460C-560C. The cracking reaction deposits carbonaceous hydrocarbons or coke on the catalyst, thereby deactivating the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, in a catalyst stripper and the stripped catalyst is then regenerated. The catalyst regenerator burns coke from the catalyst with oxygen containing gas, usually air. Decoking restores catalyst activity and simultaneously heats the catalyst to, e.g., 500C-900C, usually 600C-750C. This heated catalyst is recycled to the cracking reactor to crack more fresh feed. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Catalytic cracking has undergone progressive development since the 40s. The trend of development of the fluid catalytic cracking (FCC) process has been to all riser cracking and use of zeolite catalysts. A good overview of the importance of the FCC process, and its continuous advancement, is reported in Fluid Catalytic Cracking Report, Amos A. Avidan, Michael Edwards and Hartley Owen, as reported in the Jan. 8, 1990 edition of the Oil & Gas Journal.
Modern catalytic cracking units use active zeolite catalyst to crack the heavy hydrocarbon feed to lighter, more valuable products. Instead of dense bed cracking, with a hydrocarbon residence time of 20-60 seconds, much less contact time is needed. The desired conversion of feed can now be achieved in much less time, and more selectively, in a dilute phase, riser reactor.
Although reactor residence time has continued to decrease, the height of the reactors has not. Although the overall size and height of much of the hardware associated with the FCC unit has decreased, the use of all riser reactors has resulted in catalyst and cracked product being discharged from the riser reactor at a fairly high elevation. This elevation makes it easy for a designer to transport spent catalyst from the riser outlet, to a catalyst stripper at a lower elevation, to a regenerator at a still lower elevation.
The need for a somewhat vertical design, to accommodate the great height of the riser reactor, and the need to have a unit which is compact, efficient, and has a small "footprint", has caused considerable evolution in the design of FCC units, which evolution is reported to a limited extent in the Jan. 8, 1990 Oil & Gas Journal article. One modern, compact FCC design is the Kellogg Ultra Orthoflow converter, Model F, which is shown in FIG. 1 of this patent application, and also shown as FIG. 17 of the Jan. 8, 1990 Oil & Gas Journal article discussed above. The compact nature of the design, and the use of a catalyst stripper which is contiguous with and supported by the catalyst regenerator, makes it difficult to expand or modify such units. This means that the large, bubbling dense bed regenerator is relatively difficult to modify, in that it is not easy to increase height much. As the regenerator vessel usually is at or near grade level, it is difficult to do more than minor modifications under the regenerator.
Although such a unit works well in practice, the use of a bubbling bed regenerator is inherently inefficient, and troubled by the presence of large bubbles, poor catalyst circulation, and the presence of stagnant regions. The bubbling bed regenerators usually have much larger catalyst inventories, and longer catalyst residence times, to allow an increase in residence time to make up for a lack of efficiency.
For such units, characterized by a stripper mounted over, and partially supported by, a bubbling dense bed regenerator, there has been no good way to achieve the benefits of high efficiency regeneration, in a fast fluidized bed (FFB) region.
We studied this design, and realized that there was a way to achieve the benefits of multi-stage catalyst regeneration, at least some of which of which is efficient FFB coke combustion, while retaining most of the original design. We were able to significantly increase the coke burning capacity of these units, and provide for much drier regeneration of catalyst in the bubbling dense bed.